This invention relates to Wavelength Division Multiplex (WDM) equalizers and, more particularly, to a method of and apparatus for implementing a channelized WDM equalizer using microelectromechanical system (MEMS) devices.
Wavelength Division Multiplex (WDM) lightwave systems are the primary means of transporting telephony and data signals over long distances. Optical signals in these systems may traverse hundreds or thousands of kilometers, passing through optical amplifiers and network nodes. Wavelength-dependent amplifier gain, fiber transmission loss and optical circuit losses may result in variation or fluctuation of the signal spectrum causing degradation of weaker or more-distorted channels. Erbium-doped fiber amplifier gain spectra can be flattened using fixed filters, gain-tilt regulating optical attenuators, or by other dynamic means [1]. (Note, the numbers in brackets refers to a reference listed in the Appendix.) Recently, gain equalizers using microelectromechanical system (MEMS) variable reflectors in free-space optics have been described [2,3].
What is desired is a variable gain channelized equalizer which can be implemented using guided wave optic system components.
In accordance with the present invention, we disclose a channelized Wavelength Division Multiplex (WDM) equalizer where the gain of each WDM channel is individually controlled, enabling power adjustments of each channel over the equalizer""s entire dynamic range. The gain equalizer includes a demultiplexer with each of its outputs interfaced to a different microelectromechanical system (MEMS) reflective device which adjusts the optical power level being coupled to an optical apparatus in response to a received control signal. The channelized response enables equalization of signals that originate from diverse optical paths, either in the network or through optical multiplexers/demultiplexers, and which coalesce to a common path.
More particularly, in accordance with our invention, a wavelength division multiplex (WDM) signal equalizer comprises (1) a WDM signal guided-wave demultiplexer apparatus for receiving a WDM optical signal and demultiplexing it into a plurality of optical signal channels for output at different output ports of the demultiplexer apparatus and (2) a plurality of independently controllable microelectromechanical system (MEMS) devices, each MEMS device aligned with a different optical channel output port of the demultiplexer apparatus for adjustably controlling a signal level coupled from that optical channel output port of the WDM multiplexer apparatus to an optical apparatus in response to a control signal received at that MEMS device.
In a reflective equalizer embodiment, the signal being coupled is a reflection of the optical signal by each MEMS device back to the originating optical channel output port and the multiplexer apparatus receives the reflected adjusted optical signals and combines them into an equalized WDM signal. Another embodiment includes a circulator having an input port for receiving the input WDM optical signal, an output port for outputting the equalized WDM signal, and a third port for coupling the input WDM optical signals to and coupling the equalized WDM signal from the demultiplexer apparatus.
In a transmission equalizer embodiment, each MEMS device adjusts the coupling of the optical signal to a WDM multiplexer apparatus which receives the adjusted optical signals at a plurality of input ports and which multiplexes the adjusted optical signals together into an equalized WDM signal.
According to other features, guided-wave optical paths or optical fibers may be used to interface the demultiplexer and multiplexer to the MEMS devices. The guidedwave optical paths may be formed as part of the demultiplexer and multiplexer circuit integration. In another feature, the demultiplexer, multiplexer, optical paths, and MEMS devices may be integrated together on the same substrate.